Flyback switching power supplies are widely used in DC/DC power converters of relatively small power levels. When a flyback type of switching power supply operates in a quasi-resonant mode, the conversion efficiency may be relatively high and electromagnetic interference (EMI) may be relatively low since a power switch is turned on once the drain voltage reaches a valley value, and thus such a control method is widely used. However, valley frequency hopping can be a large problem of the quasi-resonant mode for some load characteristics. That is, the power switch can be alternately turned on at two adjacent valley values, and associated noise may be brought in when the frequency at which the power switch is alternately turned on at such valley values is in the audible frequency range.